The invention relates to electric lamps and, more specifically, to induced plasma electrodeless lamps operated at frequencies above 20 kHz.
Electrodeless fluorescent lamps have been recently introduced in markets for indoor, outdoor, industrial, and commercial applications. The advantage of electrodeless lamps is the removal of internal electrodes and heating filaments that are a life-limiting factor of conventional fluorescent lamps. Therefore, the life of electrodeless fluorescent lamps is substantially higher than that of conventional fluorescent lamps and can reach 100,000 hours.
An electrodeless fluorescent lamp introduced in the market by General Electric Corp. (GENURA) is operated at a frequency of 2.65 MHZ and used for indoor general lighting. This lamp is a replacement for a R30 incandescent lamp and has 1,100 lumen light output at 23 W of total power. The life of the GENURA lamp, 15,000 hrs, is much longer than that of the incandescent lamp. The drawback of the GENURA lamp is the high initial cost, partially due to the need to prevent electromagnetic interference and partially due to the circuit cost operating at 2.65 MHZ. Both drawbacks could be diminished if the electrodeless fluorescent lamp was operated at a frequency as low as 100 Hz.
In U.S. patent application Ser. No. 09/083,820 by Popov et al. and 09/303,951 by Chamberlain et al. (having the same assignee as the parent application hereto, Ser. No. 09/435,960), electrodeless fluorescent lamps operated at low frequencies from 50 kHz to 500 kHz were described. Those lamps utilized a ferrite core made from MnZn material, a Litz wire coil positioned about the core operated by a radio frequency oscillating driver circuit, and an aluminum cooling structure that removed heat from the reentry cavity walls and ferrite core and redirected heat to the lamp fixture. The aluminum cooling structure comprises an aluminum cylinder inside the ferrite core and an aluminum base welded to the lamp fixture. This approach and construction were found to be very effective to keep the ferrite core at a temperature below its Curie point.
However, in many lamp applications (e.g. the replacement of a conventional incandescent lamp), the large and heavy metal (aluminum or copper) base is not suitable due to its large size and weight. Also, the direct replacement of incandescent lamps requires the use of Edison-type sockets that are coupled with the base. Again, the diameter of the lamp base should not be larger than the diameter of the incandescent lamp bulb that is 60 mm.
The proximity of a ferrite core to the metal base allows the magnetic field generated by the coil and the core to intersect the metal base which results in the increase of the coil/ferrite core power losses. Indeed, the magnetic field generated by the coil induces eddy currents in the metal base that causes power losses and reduces the combined coil/structure quality factor, Q. As a result, the lamp power efficiency and hence, efficacy decreases.
The need for the incorporation of the coil oscillatory driver circuit and the associated impedance matching network in the lamp inside the lamp base and to couple them with the Edison-type socket makes the cooling issue more complex. Indeed, the temperature inside of the lamp base should not exceed around 100xc2x0 to preserve the integrity of the driver circuit components. The alternative of using of circuit components that can stand higher temperatures leads to higher costs for the driver circuit and hence, the lamp. Thus, there is a desire for a lamp with a compact base and standard socket located outside most of the internally generated magnetic field having a cooling structure limiting the temperature of circuit components therein to economically suitable values.
The present invention comprises an electrodeless fluorescent lamp that includes an envelope, often glass, containing a discharge gas (for example, a mixture of inert gas and mercury vapor) with an enclosure extending between the envelope and an Edison type socket. A ferrite core and an induction coil, typically made from Litz wire, are positioned adjacent the envelope, typically inside of reentrant cavity formed in that envelope. A cooling structure comprises a high thermal conductivity material such as a metal (typically aluminum or copper) tube positioned inside of the core and a thermally coupled high thermal conductivity material (typically ceramic) cylindrical structure that is also thermally coupled to an Edison type socket with material having a high thermal conductivity, or is formed in part as a wall portion between the envelope and the enclosure. The core extends past the tube in a direction away from the enclosure.